This invention relates to an automatic equalizer for use in equalizing a demodulated signal sent from a demodulator.
In general, a receiver section comprises a demodulator which is operable in response to a received signal to demodulate the received signal into a demodulated signal which may be a baseband signal having a binary level or a multilevel. The received signal may be subjected to an influence of fading while transmitted to the receiver section through a radio channel. The received signal has one of first and second polarizations each of which is for use in carrying a data signal. Specifically, the first polarization may be one of vertical and horizontal polarizations while the second polarization may be another one of the vertical and the horizontal polarizations. Hereinafter, the received signal of the first polarization will be called a first received signal while the received signal of the second polarization will be called a second received signal. The first and the second received signals may be transmitted to the receiver section through a co-channel which is used in common as the radio channel.
On reception of the first and the second received signals, the demodulator in the receiver section demodulates the first and the second received signal into first and second demodulated signals which carry first and second data signals, respectively.
Inasmuch as the first and the second received signals are transmitted to the receiver section through the co-channel as mentioned above, a cross polarization interference often occurs between the first received signal and the second received signal during its transmission in addition to an intersymbol interference. Taking this into account, an automatic equalizer is included in the receiver section so as to cancel the cross polarization interference.
A conventional automatic equalizer of the type described comprises a decision-feedback equalizer section, a transversal filter, and a subtracter. In case where the first demodulated signal is equalized into an equalized signal which may be produced as an output signal, the first demodulated signal is at first supplied to the decision-feedback equalizer section in order to cancel the intersymbol interference in the first demodulated signal. The decision-feedback equalizer section equalizes the first demodulated signal into a provisional equalized signal which is substantially free from the intersymbol interference. While the second demodulated signal is supplied to the transversal filter to be filtered into a filtered signal.
The provisional equalized signal and the filtered signal are further supplied to the subtracter in order to cancel the cross polarization interference. The subtracter subtracts the filtered signal from the provisional equalized signal to produce the output signal.
Such an automatic equalizer is disclosed in an article contributed by Hiroyuki Ohtsuka et al to the Transactions of the IEICE, Vol. E73, No. 3, March 1990, pages 401 to 408, under the title of "The SBS Control Algorithm of Cross Polarization Interference Canceller on Digital Radio System".
By the way, the provisional equalized signal includes a component which is caused by the cross polarization interference. As well known in the art, the provisional equalized signal appears in the decision-feedback equalizer on equalizing the first demodulated signal into the provisional equalized signal. Therefore, the intersymbol interference inevitably remains as a remaining intersymbol interference in the provisional equalized signal by the component which is caused by the cross polarization. As a result, the output signal includes a component which is caused to occur by the remaining intersymbol interference. Namely, it is difficult for the conventional automatic equalizer to effectively cancel the intersymbol interference in the output signal.